In Welsh, many verbs are followed by specific prepositions.  A comprehensive list of verb-nouns followed by their correct prepositions may be found by clicking here.

In this instance, diogelu is always followed by rhag, which means 'to safeguard from', e.g.

Mae Rhys wedi diogelu ei gyfrifiadur rhag y feirws diweddaraf.

Mae'n bwysig eich bod yn diogelu eich eiddo rhag lladron.